The present invention relates to a sol-gel process for the production of optical fiber preforms or overcladdings.
As it is well known optical fibers, widely used in telecommunications, are constituted by a central part, the so-called “core”, and a mantle surrounding the core, generally referred to as “overcladding”. A difference between the refractive index of core and overcladding in the range 0.1–1% helps confining the light in the core. This refractive index difference is obtained through different chemical compositions for core and overcladding.
Though many combinations are studied, the most common is a Germanium Oxide doped Silicon Oxide glass core (GeO2—SiO2) surrounded by a SiO2 glass overcladding. The most widely used optical fibers are of the monomodal type, having the characteristic of allowing only one light path. These fibers generally have a core diameter of about 4–8 μm and an outer diameter of the overcladding of 125 μm.
The most important parameter in evaluating the quality of a fiber is its optical loss, that is mainly due to light absorption and scattering mechanisms in the fiber and is measured in Decibel per kilometer of fiber (dB/Km).
As it is well known to those skilled in the art, UV attenuation is mainly due to absorption by cations (such as transition metal cations) present in the fiber core, while attenuation in the IR field is mainly due to absorption by —OH groups that may be present in the glass. In between, optical loss is mainly due to scattering phenomena, attributable to fluctuations of refractive index due to non-homogeneity in glass density, as well as to defects in the fiber structure, such as imperfections at the core-overcladding contact surface, bubble or cracks in the fiber, or impurities incorporated in the fiber as a result of the production process.
Optical fibers are produced by drawing a preform at temperatures around 2200° C. The preform is an intermediate product in fibers production, made up of an internal rod and an outer mantle corresponding to core and overcladding in the final fiber. Mantle to rod diameters ratio in the perform is the same as overcladding to core ratio in the final fiber. In the following, the terms rod and core will be used for the inner parts respectively of the preform and of the final fiber, while the term overcladding will be used to designate the outer portion of both preforms and fibers.
It is known that the overcladding of commercially available optical fibers is produced by several modifications of the basic Chemical Vapour Deposition (CVD) process. All the CVD-derived processes generally imply the use of gaseous mixtures comprising Oxygen (O2) and Silicon Chloride (SiCl4) or Germanium Chloride (GeCl4) in a oxyhydrogen torch to produce SiO2 and GeO2 according to the reactions:SiCl4(g)+O2(g)→SiO2(s)+2 Cl2(g)  (I)GeCl4(g)+O2(g)→GeO2(s)+2 Cl2(g)  (II)
The thus produced oxides may be deposited in particle form (so-called “soot”) on a mandrel that is then removed, or alternatively, on the inner surface of a tubular silica support that's later on drawn as a part of the final fiber.
CVD-based processes have proven capable of producing optical fibers with losses as low as about 0,2 dB/Km (at a wavelength of transmitted light of 1.55 μm), and represent the state-of-the-art in this field.
Though these production methods are quite satisfactory form the standpoint of performances, their production rates are limited thus resulting in high production costs
The sol-gel process is a chemical way to produce glasses or ceramics starting from a liquid solution, which in respect to CVD allows for a number of possible modifications, all of which however are characterized by comprising the following steps:                preparation of the sol. In this step, a precursor containing a cation the oxide of which is to be produced, is dispersed or dissolved in a liquid medium. Depending on the nature of the precursor, the liquid medium may be water, an alcohol, or a hydro-alcoholic mixture. Dispersion or dissolution of the precursor may be aided by using chemical means, such as acids, or mechanical and/or physical means, such as vigorous stirring or ultrasound agitation. In the case of silicon, commonly used precursors are alkoxides, such as Si(OCH3)4 (Tetramethylorthosilane or TMOS) and Si(OCH2CH3)4 (Tetraethylorthosilane or TEOS), or pyrogenic silica nanoparticles produced according to reaction (I) above; a commercially available example of this form of silica is Aerosil OX 50 produced by Degussa A.G. Mixed-oxides compositions may be obtained by preparing in this step a sol including precursors of more than one cation;        gelation of the sol. In this step, the precursor molecules or particles react forming a three-dimensional network of cation-oxygen bonds. The final result of this process is a porous monolith composed of an inorganic polymer including essentially the whole amount of the cation initially added in form of its precursor;        drying the wet gel. The gel produced in the former step contains in its pores all of the liquid initially present as solvent of the sol, and possibly other liquids added or produced during the process. In this step, the liquid phase in the gel pores is completely removed. This can be accomplished by normal evaporation of the liquid phase, obtaining a so-called “xerogel” or by supercritical extraction of the liquid, obtaining a so-called “aerogel”. Xerogels and aerogels differ for some physical features: xerogels have pores of diameter generally lower than pores of aerogels; also, xerogels have generally a hydrophilic surface, while aerogels have generally a hydrophobic surface. Irrespective of these differences, a dry gel (both xerogel and aerogel) essentially corresponds to the product of the CVD processes.        
The dry gels thus obtained can then be densified to the correspondent glass by suitable thermal treatments. The densification temperature of dry gels is between about 900° C. and 1500° C. at most, depending on the precursors employed and on the production process. Aerogels have generally higher densification temperatures than xerogels.
As it is well known, during the thermal treatments for complete densification of the dry gel, it is possible to include operations for its chemical cleaning. By these treatments, it is possible to take advantage of the porosity of the dry gel for gas-phase “washing” steps capable of removing organic impurities left in the gel from organometallic precursors (such as TMOS and TEOS cited before), as well as water, hydroxy-groups bound to cations in the gel network, or atoms of undesired metals.
In general, the removal of organic impurities is realized by a calcination treatment, performed by pouring an oxidizing atmosphere (oxygen or air) in the dry gel at temperatures comprised between about 200 and 800° C.
The removal of water, hydroxy-groups and foreign metals is realized by a purification treatment, pouring in the gel pores Cl2, HCl or CCl4, possibly in mixture with inert gases such as nitrogen or helium, at temperatures between about 400 and 800° C.
The last step is generally a washing treatment, realized with inert gases such as nitrogen, helium or argon, to completely remove chlorine or chlorine-containing gases from the gel pores. After these treatments, the gel is finally densified to full dense glass by heating it to a temperature higher than 900° C., and commonly in excess of 1200° C., in He atmosphere.
The above outlined washing treatments are effective in cleaning the gels to such a degree that the glasses resulting upon their densification are suitable for most applications (generally, mechanical or optical parts). However, it has been found that these treatments leave traces of gaseous compounds into the dense glass. During heating at temperatures in the range 1900–2200° C. needed to draw fibers, these traces of gaseous compounds give rise to microscopic bubbles representing centers of fracture initiation, thus leading to fiber breaks and making the prior art processes not suitable for optical fibers production.
Avoiding the formation of bubbles is the object of some patents.
From U.S. Pat. No. 4,707,174 it is known to avoid bubbles during the sintering of glass bodies produced by a sol-gel process by the addition of a fluorine compound to the porous silica body. However, fluorine is known to affect the refractive index of glasses (specifically, lowering it), so that its use may be not desirable in the production of glasses for optical applications.
U.S. Pat. No. 5,145,510 discloses that it is possible to avoid the formation of bubbles in a sol-gel derived glass by eliminating almost completely the residual —OH groups in dry gel powders, submitting these to a treatment in an atmosphere containing from 10% to 100% of steam at a temperature in excess of 1000° C. This method however is specific for sol-gel derived powders, being thus not applicable to the direct production via sol-gel of optical fiber preforms in their final form.
U.S. Pat. No. 5,236,483 discloses a method that, applied to xerogel derived glasses, allows these to be drawn into optical fibers without giving rise to the formation of bubbles. This method consists in a thermal after-treatment of the dense glass at a temperature comprised between 1500 and 2200° C. for a time comprised between 10 seconds and 5 hours, followed by a gradual cooling below 1200° C., in order not to cause mechanical stress in the final glass. The method disclosed in this patent can only be applied to xerogel-derived glasses. Xerogels, however, have the disadvantage of requiring very long times for drying (the examples in the patent show drying times of at least 7 days up to 20 days for specimens of bigger dimensions).
From the standpoint of industrial processes, it would be better to produce glasses through the aerogel route, because supercritical drying requires 2–3 days, irrespective of the dry gel body dimensions.
The process according to U.S. Pat. No. 5,236,483 shows the disadvantage, that subjecting an aerogel derived glass to the thermal treatment of U.S. Pat. No. 5,236,483 does not avoid the formation of bubbles in the following fiber-drawing operations.
It is thus the object of the present invention to provide a sol-gel based process for the production of optical fiber preforms or overcladdings suitable to be drawn yielding optical fibers with characteristics comparable to those of the state-of-the-art CVD-derived fibers.